I'm using MinGW TDM.
I guess the default build settings were 32-bit. I changed to MINGWx64 and this results in no virus detections, like you observed.
Then I changed back to MINGW (32-bit) and I get 7 detections still.
My OS is Windows 7 Pro 64-bit Service Pack 1.
UPP is Version 9251 (64-bit).
I scanned my PC with five different AV products; MSE, Malwarebytes, eset, f-secure and BitDefender (some were just the online/on-demand scanners). But they didn't find any threats on my machine. I had the 32-bit helloworld.exe running too.
The virus total site says that the scanners they are provided can have stronger heuristics than the public commercial versions.
I don't know why my 32-bit exe returns detections and yours doesn't, but it seems like nothing to do with UPP itself.
Thanks Koldo for taking the time to experiment though.